Submission #1264660

#	Time	Username	Problem	Language	Result	Execution time	Memory
1264660		mingga	Network (BOI15_net)	C++20	0 / 100	6 ms	12096 KiB

net

// Author: caption_mingle
#include "bits/stdc++.h"

using namespace std;

#define ln "\n"
#define pb push_back
#define fi first
#define se second
#define all(x) (x).begin(), (x).end()
#define sz(x) ((int)(x).size())
#define ll long long
const int mod = 1e9 + 7;
const int inf = 2e9;
const int N = 5e5 + 7;
int n, in[N], out[N], timer;
vector<int> g[N];

void dfs(int u, int p) {
	in[u] = ++timer;
	for(int v : g[u]) {
		if(v == p) continue;
		dfs(v, u);
	}
}

signed main() {
	cin.tie(0) -> sync_with_stdio(0);
	#define task ""
	if(fopen(task ".INP", "r")) {
		freopen(task ".INP", "r", stdin);
		freopen(task ".OUT", "w", stdout);
	}
	cin >> n;
	for(int i = 1; i < n; i++) {
		int u, v; cin >> u >> v;
		g[u].pb(v);
		g[v].pb(u);
	}
	dfs(1, 0);
	vector<int> vec;
	int dak = 0;
	for(int i = 1; i <= n; i++) {
		if(sz(g[i]) == 1) vec.pb(i);
		else dak = i;
	}
	sort(all(vec), [&](int x, int y) {
		return in[x] < in[y];
	});
	cout << (sz(vec) + 1) / 2 << ln;
	for(int i = 0; i < (sz(vec) - 1) / 2; i++) {
		cout << vec[i] << ' ' << vec[sz(vec) - i - 1] << ln;
	}
	if(sz(vec) & 1) {
		cout << vec[sz(vec) / 2] << ' ' << vec[0] << ln;
	}
    cerr << "\nTime: " << clock() * 1000 / CLOCKS_PER_SEC;
}

Compilation message (stderr)

net.cpp: In function 'int main()':
net.cpp:31:24: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   31 |                 freopen(task ".INP", "r", stdin);
      |                 ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
net.cpp:32:24: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   32 |                 freopen(task ".OUT", "w", stdout);
      |                 ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~

• Subtask 1: 0 / 18
• Subtask 2: 0 / 45
• Subtask 3: 0 / 37